the only requirement in a true sphere. After the ball is made
as perfect as possible with the usual turning tools, it is finished
with the tool B, made from steel tube, which is rocked to and
fro as the lathe revolves, the ball being often removed.
P. 274. Jigs.—These appliances may be constructed so as
to hold work for other operations besides drilling bolt holes, and
Fig, 791 shews two arrangements—A being a special chuck for
holding a dome cover while turning, and B a vice for supporting a
loose collar for boring.
Cutting Wheel Teeth,—The formation of the teeth of
wheels, either on paper or in the workshop, has been mentioned
at various places in this book as follows :—
Kind of Tooth.
Cutting in Metal.
Describing Teeth Curves.
pp. 175 & 1 80
pp. 510 & 517
pp. 256 & 753
A few more words on this very important subject will not be
out of place.
Spur-wheel Teeth.—A milling cutter is always used to remove
the interspaces, but the * blank' to be cut may be mounted in
one of various ways. On p. 180, the dividing heads are used for
support, but this method is only suitable for small diameters.
When wheels of large diameter form the regular work of a shop,
special machines are adopted; in many cases so constructed as to-
automatically rotate the wheel through the pitch arc after every
cut. Such a machine is shewn in Fig. 792, elaborate but effective,
where A is the cutter, B the work, c the dividing wheel, and D a.
sector for tilting the table to suit bevel gears.
Yet another method, shewn in Swasey's machine, Fig. 793, is
based on the fact that if a number of wheels be cut so as to each
gear with a given rack, they will all gear the one with the other.